It has been common practice in recent years to use helical or spiral conveyors in connection with continuous food freezers. The helical or spiral configuration provides a substantial length of conveyor within which the freezing of the food may be accomplished in a more restricted space than would be possible with a straight, in-line type of conveyor freezer. These helical conveyor freezers typically included an insulated chamber or enclosure with an entrance opening located in the sidewall toward the bottom of the enclosure. The exit opening is located in a wall opposite the entrance opening but toward the top of the enclosure. The helical conveyor is centrally located in the enclosure, having a vertical axis on which a cylindrical drum is positioned to drive the conveyor. The superimposed flights or tiers of the conveyor transfer the food products from an entrance section, which extends from the entrance opening to the bottom flight, where the food products move upwardly on successive flights and discharge on an exit section that extends from the uppermost flight out through the exit opening.
While the mechanical means for supporting the conveyor varies to some extent, it basically consists of an open frame positioned around the helical conveyor with support members extending inwardly to mount helically disposed rails which support a flexible, open mesh belt. The inner edge of the belt rests against the rotating driven drum which causes the various flights of the conveyor to rotate and progress upwardly on the supporting rails.
In the prior art of helical conveyor food freezers, there are different means used to freeze the food products. In the earlier helical conveyor food freezers, the freezing means was typically a mechanical refrigeration system having evaporator coils within the insulated enclosure to cool air circulated therein across the food products. There are also examples in the prior art of helical conveyor food freezers which utilize cryogenic cooling in the form of sprayed liquid nitrogen or liquid CO.sub.2. The cryogenic freezers use various combinations of liquid cryogen, condensed vapor as snow or cryogen vapor to cool or freeze the food products.
U.S. Pat. No. 3,412,476 to Astrom is directed to a helical conveyor food freezer having mechanical refrigeration and arrangements of cylindrical baffles coaxial with the conveyor to direct the cooling medium through the flights of the conveyor in a direction opposite to the movement of the foods products. U.S. Pat. No. 3,315,492 to Dreksler discloses a helical conveyor freezer having mechanical refrigeration and fan means for directing the air flow downwardly into the core of the conveyor and outwardly across the food products on the various flights of the conveyor.
U.S. Pat. No. 4,023,381 to Onodarow discloses a helical conveyor food freezer having mechanical refrigeration and fan means located in the core or center of the helical conveyor arranged to circulate cooling air horizontally and diametrically with respect to the conveyor, with generally cylindrical baffle means enclosing the conveyor to maintain the cooling medium therein. U.S. Pat. No. 4,056,950 to Kaufmann, Jr. is similar to the Onodarow patent in showing a continuous helical conveyor food freezer having mechanical refrigeration and generally horizontal air flow across the helical conveyor. U.S. Pat. No. 4,426,093 to Voitko also discloses a continuous helical conveyor freezer having mechanical refrigeration and air flow which is directed generally downwardly through the flights of the vertical conveyor.
U.S. Pat. No. 4,612,780 to Briley et al. similarly shows a continuous helical conveyor having cooling air flow across and through the helical conveyor as the air moves generally downwardly. Cylindrical baffles and conical baffles are used to direct the air flow in the Briley et al. patent. Briley et al. also teaches reversal of the air flow, using up flow, with the products moving either upwardly or downwardly on a helical conveyor. U.S. Pat. No. 4,798,062 to Lipinski et al. shows a continuous helical conveyor freezer having mechanical refrigeration means and including baffle means to direct air flow downwardly through the core of the conveyor and upwardly through the flights of the conveyor.
U.S. Pat. No. 4,875,343 to Jeppsson discloses a continuous helical conveyor freezer having mechanical refrigeration and disclosing many different embodiments insofar as air flow is concerned. In the embodiment of FIG. 2, the air flow is generally downwardly in the core of the conveyor and radially outwardly through the various flights of the conveyor, the air dividing, passing upwardly and downwardly through the conveyor and back to the air circulation means. FIGS. 14, 15 and 16 show alternative air flow patterns in which the cooling media passes upwardly through the entire length of the helical conveyor or enters the side and moves horizontally around the periphery of the conveyor or enters from the outside and moves upwardly and downwardly from the center part of the conveyor, as shown in FIG. 16.
There are various prior art patent disclosures of cryogenically cooled helical conveyor food freezers of less relevance to the present invention; these patents are noted as including the patents to Chamberlain et al., U.S. Pat. No. 4,078,394; Harrison U.S. Pat. No. 3,866,432; Styley Jr. et al., U.S. Pat. No. 4,739,623; and Loades et al, U.S. Pat. No. 5,020,330.
In the fast freezing of food products, there is a problem of food dehydration during the freezing process. The heat exchange medium engaging the product is often air which has little moisture, and therefore tends to pick up moisture from the food. Accordingly, it is important to contact the food products with a sufficient amount of the cooling medium to drop the temperature of the food quickly and form a crust on the outer surface of the food products to prevent the further loss of moisture. Moisture loss is undesirable from two standpoints. First, it reduces the weight of the product, which since the product is sold by the pound, reduces its value. In addition, the dehydration of the food generally results in a deterioration in the quality of the product. For these reasons, it is desirable to minimize the time period in which the food products are cooled to the point of forming an outer crust which then greatly reduces the further loss of moisture.
The most rapid cooling is known to be accomplished by cryogenic cooling using liquid CO.sub.2 or nitrogen, to obtain the greatest difference in temperature between the food products and the cooling medium. Cryogenic cooling is also preferred since it has much less dehydrating effect than does the dry air resulting from mechanically refrigerated systems. However, the deterrent to the universal application of cryogenic cooling to all food-freezer applications is the fact that it, in most instances, is considerably more expensive than the use of mechanical refrigeration. In an effort to obtain a balance between operating cost and performance, there have been continuous food freezers which employ a combination of mechanical refrigeration and cryogenic cooling. One such freezer is disclosed in copending U.S. application Ser. No. 704,806, filed May 23, 1991, and assigned to the same Assignee as the present application.
There are means of improving the efficiency of continuous food freezers utilizing mechanical refrigeration or combinations of mechanical refrigerations and cryogenic cooling. In order to quickly cool incoming food products, it is important that a continuous type food freezer be arranged to deliver the cooling medium to the incoming food products at its lowest temperature. In addition, to operate the mechanical refrigeration means at the most efficient level, it is desirable to extract as much heat as possible from the cooling media before it is recirculated back to the heat exchange coils of the mechanical refrigeration apparatus. It would be desirable to provide air flow within a continuous helical conveyor food freezer, to satisfy both of the abovedescribed objectives.